A new category of primitive hemopoietic stem cells determined in murine bone marrow long-term culture (CFU-BMLTC) is described. The novel precursors give rise to hemopoietic colonies corresponding to "cobble stone" areas observed on the adherent cellular layer. The linear relationship between the colony number and the dose of bone marrow cells explanted in the culture has been discovered. Radiosensitivity of the new type of hemopoietic precursors in the bone marrow of adult mice is characterized by D0-1.28 Gy. CFU-BMLTC were also identified in 13-day mouse fetal liver. In contrast to CFUs the precursors described are less sensitive to extensive in vivo treatment with hydroxyurea. The data obtained have permitted the authors to make a conclusion on a higher status of CFU-BMLTC as compared to CFUs in the hemopoietic stem cell hierarchy.